Polycrystalline diamond

ABSTRACT

The invention relates to a polycrystalline diamond (PCD) body comprising PCD material having a graphitisation onset temperature, the PCD body having a working surface and comprising a first region remote from the working surface, the first region containing a catalysing material; and a second region extending a depth from the working surface into the PCD body, the second region being substantially free of catalysing material; the depth having a thermal gradient characteristic that when the temperature at the working surface is 900 degrees centigrade, the temperature at the depth is in the range from 780 degrees centigrade to 850 degrees centigrade and to inserts, machine tools and drill bits comprising the PCD body.

FIELD

The invention relates to polycrystalline diamond (PCD) bodies,particularly but not exclusively for use in boring into the earth, highwear applications, machine tools, mining or other degradation of hardmaterials. More particularly, the PCD bodies are for use in applicationsin which resistance to thermal degradation of the PCD bodies is valued.

BACKGROUND

Polycrystalline diamond (PCD) is an example of a superhard, also calledsuperabrasive, material comprising a mass of substantially inter-growndiamond grains, forming a skeletal mass defining interstices between thediamond grains. PCD material typically comprises at least about 80volume % of diamond and may be made by subjecting an aggregated mass ofdiamond grains to an ultra-high pressure of greater than about 5 GPa andtemperature of at least about 1,200° C. in the presence of a sinteringaid. Suitable sintering aids for PCD may also be referred to as asolvent/catalyst material for diamond. Solvent/catalyst material fordiamond is understood be material that is capable of promoting directinter-growth of diamond grains at a pressure and temperature conditionat which diamond is thermodynamically more stable than graphite, and ofpromoting the conversion of diamond to graphite at ambient pressure.Examples of solvent/catalyst materials for diamond are cobalt, iron,nickel and certain alloys including any of these. PCD may be formed on acobalt-cemented tungsten carbide substrate, which may provide a sourceof cobalt solvent/catalyst for the PCD. The interstices within PCDmaterial may be wholly or partially filled with binder or fillermaterial, which may be solvent/catalyst material.

Components comprising PCD are used in a wide variety of tools forcutting, machining, drilling or degrading hard or abrasive materialssuch as rock, metal, ceramics, composites and wood-containing materials.For example, PCD bodies are commonly used as cutter inserts on drillbits used for boring into the earth in the oil and gas drillingindustry. PCD bodies are also used for machining and millingmetal-containing bodies, such as may be used in the auto manufacturingindustry. In many of these applications the temperature of the PCDmaterial becomes elevated as it engages a rock formation, workpiece orbody with high energy. Examples of PCD composite abrasive compacts aredescribed in U.S. Pat. Nos. 3,745,623; 3,767,371 and 3,743,489.

PCD is extremely hard and abrasion resistant, which is the reason it isthe preferred tool material in some of the most extreme machining anddrilling conditions, and where high productivity is required. Adisadvantage of PCD containing a solvent/catalyst material for diamondas a filler material may be its relatively poor thermal stability aboveabout 400° C.

U.S. Pat. No. 6,878,447 discusses that, due to the presence of thebinder-catalysing material, diamond is caused to graphitise astemperature increases, which typically limits its use to operationtemperatures of up to about 750 degrees centigrade. The patent disclosesa polycrystalline diamond element comprising a body with a workingsurface, wherein a first volume of the body remote from the workingsurface contains a catalysing material, a second volume of the bodyadjacent to the working surface is substantially free of the catalysingmaterial to a depth from the working surface, wherein a thermal gradientof the bonded diamonds causes a 950 degrees centigrade temperature atthe working surface to be less than 750 degrees centigrade at the depth.

U.S. Pat. No. 7,377,341 discloses a thermally stable ultra-hard compactconstruction comprising a body formed from an ultra-hard material, suchas diamond, having a thermally stable region positioned adjacent aworking surface of the body, and a metallic substrate connected to thebody. The thermally stable region may be formed from a material selectedfrom the group consisting of consolidated materials that are thermallystable at temperatures greater than about 750 degrees centigrade, oreven greater than about 1,000 degrees centigrade and that aresubstantially free of a catalyst material. The body and metallicsubstrate are joined together by high pressure/high temperature process.

SUMMARY

The purpose of the invention is to provide a thermally stable PCD bodyhaving enhanced impact toughness and reduced manufacturing cost. Moreparticularly, the purpose is to provide a PCD body comprising athermally stable stratum that is as thin as possible while maintainingkey benefits of having a thermally stable stratum.

As used herein, a “working surface” of a body is any part of the bodywhich may in use contact a workpiece or other body being worked. It isunderstood that any portion of a working surface is also a workingsurface.

As used herein, the “graphitisation onset temperature” is the lowesttemperature at which at least 10 percent by mass of the diamond withinat least a portion of a given PCD body converts into a form ofnon-diamond carbon, such as graphitic carbon, after thirty minutes atthe temperature.

As used herein, “catalysing material” means a material that is capableof promoting the sintering and inter-bonding of diamond grains to formpolycrystalline diamond material at a temperature and pressure at whichdiamond is thermodynamically more stable than graphite. Examples ofcatalysing material are cobalt, iron, nickel and manganese, and certainalloys including any of these.

As used herein, the phrases “substantially free” or “substantialabsence” in relation to catalysing material within a PCD body means thata volume or the interstices within a volume are substantially but notnecessarily completely devoid of catalysing material, any content ofcatalysing material that may be present being sufficiently low that itdoes not materially promote the degradation of a region of PCD in whichit is present. Some of the diamond surfaces within the volume may stillbe in contact with catalyst material remaining within the proximateinterstices.

A first aspect of the invention provides a polycrystalline diamond (PCD)body comprising PCD material having a graphitisation onset temperature,the PCD body having a working surface and comprising a first regionremote from the working surface, the first region containing catalysingmaterial, and a second region extending a depth from the working surfaceinto the PCD body, the second region being substantially free ofcatalysing material; the depth having a thermal gradient characteristicthat when the temperature at the working surface is about 900 degreescentigrade, the temperature at the depth is in the range from about 780degrees centigrade to about 850 degrees centigrade, or a temperature atthe depth in the range from about 800 degrees centigrade to about 820degrees centigrade, or even a temperature at the depth within 10 degreescentigrade eitherside of the graphitisation onset temperature.

Embodiments of the invention have the advantage that the substantialabsence of catalysing material within the PCD body volume defined by thedepth (the second region) thus determined results in significantlyimproved thermal stability of the PCD body (compact) proximate theworking surface while avoiding excessive and unnecessary removal ofcatalysing material to a greater depth. Such excessive removal addsunnecessary cost to the manufacturing process and is believed to resultin inferior impact strength.

In one embodiment, the depth is in the range from about 20 to about 90microns from the working surface, or the depth is in the range fromabout 40 to about 70 microns from the working surface.

In one embodiment, bonded diamond grains of the PCD body have a sizedistribution in which more than one peak or mode is evident. In oneembodiment, the bonded diamond grains have a size distributioncharacteristic that at least 20% of the grains have an average sizegreater than 10 microns, and at least 10% of the grains have an averagesize in the range from 10 to 20 microns. In one embodiment, the bondeddiamond grains have an average size in the range from 5 microns to 20microns.

Embodiments of the invention have the advantage of enhanced working lifeand cutting or penetration rate of the PCD body in rock drilling orearth boring applications, and shear cutting rock drilling inparticular.

A second aspect of the invention provides an insert for a machine toolor drill bit, the insert comprising an embodiment of a PCD bodyaccording to the present invention.

In one embodiment, the insert comprises a PCD body joined to a cementedcarbide substrate, and in one embodiment, the PCD body is part of acutter insert for use in boring into the earth, particularly fordrilling oil and gas wells. In another embodiment, the PCD body is partof an insert for use in pavement degradation, mining, machining,including turning, milling, drilling or in certain other wearapplications.

A third aspect of the invention provides a machine tool or drill bitcomprising an embodiment of an insert according to the second aspect ofthe present invention.

Embodiments of the PCD bodies according to the present invention havethe advantage that they exhibit extended operating lives or cuttingrates.

DRAWINGS

Non-limiting embodiments will now be described with reference to thefigures, of which

FIG. 1 shows a schematic longitudinal cross sectional view of anembodiment of a PCD body, as well as a magnified view of a part of thecross section.

FIG. 2 shows a schematic longitudinal cross sectional view of anembodiment of a PCD body, as well as a magnified view of a part of thecross section.

FIG. 3 shows a frequency graph of number of diamond grains versus grainsize for an embodiment of a PCD body.

The same reference numbers refer to the same features in all drawings.

DETAILED DESCRIPTION OF EMBODIMENTS

Where meanings of terms used herein are not sufficiently clear, a termor terms should be understood to have the same meaning as in U.S. Pat.No. 6,878,447.

With reference to FIG. 1 and FIG. 2, embodiments of PCD inserts 10 eachcomprise a PCD body 16 comprising PCD material, the bodies integrallybonded to respective cobalt-cemented tungsten carbide substrates 18, thePCD material having a respective graphitisation onset temperature, andthe PCD body 16 having a working surface 13. A first region 12 of thePCD body 16 remote from the working surface 13 contains a catalysingmaterial (not shown) and a second region 11 of the body 16 adjacent tothe working surface 13 is substantially free of catalysing material to adepth 14 from the working surface 13. The depth 14 is such that when aportion of the working surface 13 is heated to 900 degrees centigrade,the temperature at the depth 14 is within ten degrees centigrade of thegraphitisation onset temperature of the PCD material and in the rangefrom 780 to 850 degrees centigrade. In one embodiment, the depth is inthe range from about 60 microns to about 80 microns.

With reference to FIG. 3, an embodiment of a PCD body comprises PCDmaterial in which the sintered diamond grains have a size distributionthat can be resolved into more than one distribution, each having asingle different peak or mode. Such distributions may be referred to asmultimodal distributions. In this embodiment, the mean size of thediamond grains is in the range from about 5 to about 20 microns and thesize distribution can be resolved into at least three distinct peaks.

The size distribution of the diamond grains within PCD material ismeasured by means of image analysis carried out on a polished surface ofa PCD body. A Saltykov correction may be applied to convert the sizedistribution obtained from the two-dimensional image data to a particlesize distribution in three dimensions, as is standard practice. As usedherein, particle size is in terms of the equivalent circle diameter ofthe particle, determined as the diameter of a circle having an areaequal to the area of a shape of a cross section of a particle.

The polycrystalline diamond element may be made using an ultra-highpressure sintering method as is well known in the art, in which anaggregate mass of diamond particles is subjected to a pressure of atleast about 5.5 GPa and a temperature of at least about 1,250 degreescentigrade in the presence of a catalysing material. During this step,catalysing material such as molten cobalt is allowed to infiltrate intoan aggregate mass of diamond grains, which may be formed into anunbonded, or at least a weakly bonded, porous pre-form comprisingdiamond grains. The catalysing material infiltrates and fillssubstantially all of the pores or interstitial regions within theaggregate mass, promoting the intergrowth of the diamond grains to forma sintered PCD body. After the sintering step, catalysing material isremoved from a region adjacent a working surface of the PCD body by anyof various means known in the art, for example by using an acid liquor(e.g. hydrofluoric acid, nitric acid or mixtures thereof) to leach outcatalysing material. Other exemplary methods are disclosed ininternational patent publication number WO2007042920, and in SouthAfrican patent publication number 2006/00378.

The graphitisation onset temperature is found by subjecting each of aset of samples of a type of PCD body (element) to heat treatment atvarious temperatures within the range from about 760 degrees centigradeto about 850 degrees centigrade in air for thirty minutes. It isimportant that the samples are substantially identical to each other interms of the nature and size distribution of diamond grains, the typeand content of solvent/catalyst material, sinter quality, and weremanufactured using substantially the same process and processconditions. It is also important that the PCD body has at least aportion from which the solvent/catalyst has not been removed ordepleted, since this is the portion that must be inspected and analysedfor evidence of conversion of diamond to graphite. It is not necessaryto deplete any portion of the samples of solvent/catalyst material whencarrying out the process of determining the graphitisation onsettemperature, and in general it is preferable not to do so. After eachheat treatment, the treated sample is removed and subjected tomicro-Raman analysis to determine whether there is evidence that diamondgrains, or at least portions of diamond grains have converted tographite or other non-diamond carbon. Micro-Raman spectroscopy issuitable for measuring the levels of diamond, graphite and othernon-diamond carbon. Graphite appears as a distinct peak at about 1,580inverse centimetres and other non-diamond carbon appears as a peak tothe left of the diamond peak. In some cases, this will be evident frommere visual inspection. If more than about 10 volume percent of thediamond within conversion has occurred, this means that temperature usedin the heat treatment of that sample exceeded the critical degradationtemperature (graphitisation onset temperature). It has been found thatthe temperature range over which a very substantial portion of thediamond converts to graphite is rather narrow, and was about 15 degreescentigrade in one example. The amount of diamond to graphite conversionover about 20 degrees centigrade may be so extensive that very little,if any diamond can be detected within a portion of the heat treatedsample.

Once the graphitisation onset temperature has been determined for agiven grade or type of PCD body, the depth from a working surface fromwhich solvent/catalyst material should be depleted must be determined.This may be done by means of a wear test known in the art. For example,a well-known test is to engage a cutting edge of an element with aworkpiece of rock or other abrasive material moving at high speedrelative to the element, and so to effect a cutting action on theworkpiece. The time period of the engagement may be varied depending onthe type of workpiece material and its speed of relative movement, amongother factors, as would be appreciated by the person of ordinary skill.Such tests are known to be carried out in various ways, such as “logturning”, in which a generally cylindrical workpiece is caused to rotateabout its cylindrical axis on a lathe, for example, or a “verticalborer” test, in which the workpiece has a generally disc-like form andis caused to rotate about a substantially vertical axis. The action ofcutting the workpiece results in the wear of the element proximate thecutting edge and the formation of a wear scar, the dimensions of whichmay readily be measured. The depth of the wear scar may be used as anindicator of the wear resistance and likely performance of an element inapplication, all else being equal.

A wear test of the kind described above and which is known in the artmay be used to determine whether a depth associated with a second volumeaccording to the invention has been achieved within a polycrystallinediamond body (element). For example, a (body) element having a secondvolume from which catalyst material has been substantially removed ordepleted to a depth may be subjected to a wear test wherein the relativespeed of the workpiece is increased to the point where the temperatureor proximate the cutting edge is 900 degrees centigrade. The achievementof such a temperature may be determined by observation of the colour ofthe light emitted from the cutting edge, which glows white-hot at thistemperature. A more accurate measurement of the temperature may beachieved by means of an optical pyrometer, which is a device thatmeasures temperature by analysing the frequencies of light given off byglowing-hot bodies. After the wear test, once the cutting edge has beenallowed to cool down, the element is removed from the test apparatus andthe wear scar dimensions, its length and depth in particular, aremeasured. It has been found that wear scar dimensions are substantiallygreater for elements having a second volume depth less than thataccording to the invention. This is believed to be because a firstportion proximate the second volume at the depth is degraded byconversion of diamond to graphite or other non-diamond carbon attemperatures above about 815 degrees centigrade. Evidence of suchdegradation or conversion may be measured at or proximate the depth bymeans of Raman or micro-Raman spectroscopy, or may be evident fromvisual inspection under a microscope, and provides an indication thatthe depth of the tested sample is too shallow, since the temperature atthe depth during the test was too high. In order to determine a correctdepth, the test should be repeated using elements having variousdifferent depths, using a trial-and-error proximation approach.

Unexpectedly, it has been found that there is not a single value for thegraphitisation onset temperature corresponding to all PCD bodies, andthat the this temperature depends on factors such as the sizedistribution of the diamond grains, the type and content of thecatalysing material and the quality of the sintering of the diamondgrains. It believed that certain improved PCD bodies as have beendeveloped in the recent past have higher graphitisation onsettemperature than previous PCD bodies.

An embodiment of the invention is described in more detail withreference to the example below, which is not intended to limit theinvention.

EXAMPLE

Fifteen polycrystalline diamond (PCD) bodies (compacts) havingmultimodal diamond grain size distribution as shown in FIG. 3 weremanufactured in a conventional way using a ultra-high pressure, hightemperature sintering process. The compacts were substantiallycylindrical in shape, having a diameter of about 16 mm. The compactscomprised a layer of PCD integrally bonded onto a cobalt-cementedtungsten carbide (WC) substrate, the PCD layers being 2.2 mm thick. Thediamond content of the PCD layer was about 92% by volume, the balancebeing cobalt and minor precipitated phases such as WC.

The input diamond powder was prepared by blending diamond powders havingdifferent average sizes, the size distribution of the grains within theresulting blended powder have the size distribution characteristic that9.77 wt. % of the grains had average grain size less than 5 microns,7.64 wt. % of the grains had average size in the range from 5 microns to10 microns, and 82.58 wt. % of the grains had average grain size greaterthan 10 microns. The blended powder was formed into an aggregated mass,which was integrally bonded to a cobalt-cemented tungsten carbide (WC)substrate during the sintering step, as is conventional in the art.During this step, cobalt from the substrate infiltrated the aggregatedmass of diamond grains, filling pores between them and promoting theintergrowth of the diamond grains, resulting in a polycrystallinediamond compact bonded to a substrate. The diamond grains within the PCDthus produced had a multimodal size distribution having thecharacteristic that 34.7 wt. % of the grains had average grain size lessthan 5 microns, 40.4 wt. % of the grains had average size in the rangefrom 5 microns to 10 microns, and 24.9 wt. % of the grains had averagegrain size greater than 10 microns. The grain size distribution of thesintered PCD is different from that of the input grains due to mutualcrushing of the grains at high pressure, in addition to the shifttowards coarser grain sizes that normally occurs during the sinteringprocess.

Polycrystalline diamond bodies (compacts) comprising diamond grains ahaving size distribution with more than one peak, each corresponding toa respective “mode”, have been found to have advantageous properties.So-called “multimodal” PCD is disclosed in U.S. Pat. Nos. 5,505,748 and5,468,268. Multimodal polycrystalline bodies are typically made byproviding more than one source of a plurality of grains, each sourcecomprising grains having a substantially different average size, andblending together the grains or particles from the sources. Measurementof the size distribution of the blended grains typically revealsdistinct peaks corresponding to distinct modes. The blended grains arethen formed into an aggregate mass and subjected to a sintering step athigh or ultra-high pressure and elevated temperature, typically in thepresence of a sintering agent. The size distribution of the grains isfurther altered as the grains are compacted against one another andfractured, resulting in the overall decrease in the sizes of the grains.Nevertheless, the multimodality of the grains is usually still clearlyevident from image analysis of the sintered article.

Nine of the compacts were used to determine the minimum graphitisationtemperature. The PCD layer was detached from the carbide substrate ofeach of the nine compacts by means of electro-discharge machining (EDM),yielding nine un-backed PCD discs, each about 2.2 mm thick. Each of thePCD discs was treated in air for thirty minutes at a differenttemperature in the range from 650 degrees centigrade to 850 degreescentigrade, specifically at 650, 700, 730, 750, 770, 800, 815, 825 and850 degrees centigrade. After heat treatment, each sample was fracturedto expose a cross-section, the exposed cross-section then being visuallyexamined and subjected to micro-Raman analysis in order to detect thepresence of diamond and/or graphite. An external surface of each samplewas similarly examined to determine whether a different result would beobserved for the fracture and external surfaces.

In the cases of all samples treated at temperatures in the range of 650degrees centigrade to 800 degrees centigrade, no evidence of graphitewas detected and the diamond had clearly not degraded to any significantdegree. However, substantially all of the diamond had converted tographite in the samples treated at temperatures in the range from 815degrees centigrade to 850 degrees centigrade. This indicated that theminimum graphitisation temperature was in the range from 800 degreescentigrade to 815 degrees centigrade.

The PCD faces or end working surfaces (i.e. the flat end faces of thePCD, coterminous with the working end of the cylindrical compacts) ofeach of six of the remaining compacts were then polished and treatedwith acid to remove substantially all of the cobalt from differentrespective depths. This was done by masking the major portion eachcompact by suitable means as is known in the art, leaving only thatportion exposed from which the cobalt was to be removed. The maskedsamples were bathed in hot hydrofluoric/nitric acid for about 3.5 hoursat a temperature close to the boiling point of the acid under ambientpressure, which was sufficient to remove substantially all of the cobaltfrom the exposed portion. A different volume of PCD was exposed for eachsample, the depth of the exposed portion being different in each case.This resulted in the cobalt in each sample being removed to a differentdepth from the flat PCD face, the depths being in the range from 40microns to 140 microns in steps of about 20 microns. Six “leached”compacts were thus produced.

A further ten PCD compacts were used to establish suitable wear testconditions under which to determine which of the leached samplesdisplayed the required thermal character proximate a leached workingsurface. The wear test involved causing a PCD compact to engage arapidly rotating log of sandstone mounted onto a lathe, a working edge(surface) of the PCD cutting into the sandstone. As is typical in suchtests, the engaged working edge glowed red hot initially. The speed ofrotation of the sandstone log was increased to the point at which theworking edge glowed white hot, indicating that 950 degrees centigradehad been achieved.

Once the required turning speed had been determined, each of the sixleached PCD compacts was subjected to the wear test for three minutesusing that speed. After each test, the PCD at the leached depth wasvisually inspected and subjected to micro-Raman analysis in order todetect the presence of substantial graphite. Using this method, thesample with the optimum leach depth was identified, this depth being inthe range from 40 to 60 microns.

Further compacts were the prepared having a leach depth of 50 micronsfrom the flat working surfaces and subjected to standard performancewear tests.

Unfortunately it was not possible to compare directly the performance ofthese compacts with those having the leached depth characteristicdisclosed in the prior art, since the prior art asserts that thetemperature at the depth should be less than 750 degrees centigrade.Since the graphitisation temperature for the PCD grade used in thisexample was found to be in the range from 800 degrees centigrade to 815degrees centigrade, there was no marker or means of identifying a depthat which 750 degrees centigrade was achieved. Nevertheless, theperformance wear test results of the optimum compacts made according tothis example provided qualitative positive indication that they were atleast as effective as the best available in the art, while having theshallower leach depth than any compact known in the art.

1. A polycrystalline diamond (PCD) body comprising PCD material having agraphitisation onset temperature, the PCD body having a working surfaceand comprising a first region remote from the working surface, the firstregion containing catalysing material; and a second region extending adepth from the working surface into the PCD body, the second regionbeing substantially free of catalysing material; the depth having athermal gradient characteristic that when the temperature at the workingsurface is 900 degrees centigrade, the temperature at the depth is inthe range from 780 degrees centigrade to 850 degrees centigrade.
 2. APCD body as claimed in claim 1, in which the temperature at the depth inthe range from 800 degrees centigrade to 820 degrees centigrade.
 3. APCD body as claimed in claim 1, in which the temperature at the depth iswithin 10 degrees centigrade eitherside of the graphitisation onsettemperature.
 4. A PCD body as claimed in claim 1, in which the depth isin the range from 20 to 90 microns from the working surface.
 5. A PCDbody as claimed in claim 1, in which the bonded diamond grains have asize distribution in which more than one peak or mode is evident.
 6. APCD body as claimed in claim 1, in which bonded diamond grains of PCDbody have a size distribution characteristic that at least 20% of thegrains have an average size greater than 10 microns, and at least 10% ofthe grains have an average size in the range from 10 to 20 microns.
 7. APCD body as claimed in claim 1, in which the bonded diamond grains havean average size in the range from 5 microns to 20 microns.
 8. An insertfor a machine tool or drill bit, comprising a PCD body as claimed inclaim
 1. 9. A machine tool or drill bit comprising an insert as claimedin claim 8.